Amorphous alloy catalysts for conversion of exhaust gases to harmless gases

ABSTRACT

This invention relates to a catalyst for cleaning exhaust gas, which is prepared by dipping in hydrofluoric acid an amorphous alloy composed of 20 to 80 atom % of one or more elements selected from the group consisting of Nb, Ta, Ti and Zr (the sum of Nb and Ta being up to 70 atom %), 0.5 to 20 atom % of one or more elements selected from the group consisting of Ru, Pd, Rh, Pt and Ir, and the balance of one or two of Ni and Co. This catalyst can clean at a low temperature an exhaust gas containing NO or CO discharged from apparatuses or engines wherein various organic substances are burnt.

TECHNICAL FIELD

The present invention relates to highly active catalysts for conversionof combustion exhaust gases to harmless gases; catalysts being easilyproduced and recovered by simple methods.

BACKGROUND ARTS

Apparatuses and engines, burn various organic substances, exhaust gasescontaining NOx, CO, unburnt hydrocarbons, etc. Such exhaust gases haveso far been converted basically by reactions to change carbon compoundsinto carbon dioxide and water and to change nitrogen oxides intonitrogen gas, using catalysts of platinum group elements carried byceramics.

One of the present inventors filed an application for patentregistration of amorphous alloy electrode materials containing Ni-Ta-Ptgroup metals as essential components, for generation of oxygen gas inthe hydrolysis of aqueous solutions, under Japanese Patent Kokai No.61-281889.

Furthermore, three of the present inventors filed applications forpatent registration of surface activated amorphous alloys as electrodesfor electrrolyzing solutions, comprising one or more elements of Ti, Zr,Nb and Ta, and Ni and platinum group metals, and also of an activationtreatment process for them under Japanese Patent Kokai Nos. 62-96633,62-96634 and 62-96636 and moreover filed an application for patentregistration of similar surface activated supersaturated solid solutionalloys for electrodes for electrolyzing solutions and also of anactivation treatment process for them under Japanese Patent Kokai No.62-96635.

In addition, the present inventors found surface activated amorphousalloys for methanol based fuel cells, and filed an application forpatent registration under Japanese Patent Kokai No. 63-11647.

The conventional catalysts with platinum group elements carried byceramics used for conversion of the exhaust gases from variousapparatuses and engines burning various organic substaces are high inreaction temperature and difficult to recover expensive platinum groupelements.

Therefore, the development of any catalysts which can act at lowtemperatures to convert low temperature exhaust gases in the beginningof combustion, are high in activity and can be easily regenerated hasbeen awaited.

DISCLOSURE OF THE INVENTION

The present invention relates to catalysts for conversion of exhaustgases to harmless gases, comprising an amorphous alloy consisting of20-70 at % of at least one element selected from Nb and Ta, 0.5-20 at %of at least one element selected from Ru, Pd, Rh, Pt and Irt, with thebalance being substantially at least one element selected from Ni andCo, having been subjected to activation treatment by immersion inhydrofluoric acid; catalysts for conversion of exhaust gases to harmlessgases, comprising an amorphous alloy consisting of 20-80 at % of atleast one element selected from Ti and Zr, 0.5-20 at % of at least oneelement selected from Ru, Pd, Rh, Pt and Ir, with the balance beingsubstantially 10 at % of at least one element selected from Ni and Co,having been subjected to activation treatment by immersion inhydrofluoric acid; and catalysts for conversion of exhaust gases toharmless gases, comprising an amorphous alloy consisting of at most 70at % of at least one element selected from Nb and Ta, and 20-80 at % inthe sum of at least one element selected from Nb and Ta and at least oneelement selected from Ti and Zr, 0.5-20 at % of at least one moreelement selected from Ru, Pd, Rh, Pt and Ir, with the balance beingsubstantially at least 10 at % of at least one element selected from Niand Co, having been subjected to activation treatment by immersion inhydrofluoric acid.

The present invention provides catalysts which can convert gasescontaining NOx, CO, unburnt hydrocarbons, etc. from apparatuses andengines using various organic fuels to harmless gases based on thereactions changing carbon compounds into carbon dioxide and water, andnitrogen oxides into nitrogen at temperatures close to room temperature,and the catalysts have high activity and easily produced andregenerated.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic sectional view showing apparatus for producing thealloys of the present invention as an example.

THE BEST CONFIGURATION FOR EXECUTING THE INVENTION

Usually an alloy is crystalline form in the solid state. However, if analloy with a specified composition is rapidly solidified from the moltenstate or produced in any other way to avoid the formation of long-rangeatomic ordering at solidification, an amorphous structure similar to aliquid structure different from crystalline structure can be obtained,and such an alloy is called an amorphous alloy. An amorphous alloy isusually a homogeneous single-phase alloy of a supersaturated solidsolution, and has a very high mechanical strength compared withconventional practical metals, exhibiting various properties such asabnormally high corrosion resistance, depending on its composition.

On the other hand, the conventional catalysts for the conversion ofexhaust gases to harmless gases are usually effective at hightemperatures, and any catalysts which are capable of converting lowtemperature harmful gases in the beginning of combustion or lowconcentration harmful gases from fossil fuel burning equipments onhouses to harmless gases had not been developed.

In this regard, for the purpose of obtaining metallic materials withhigh electrocatalytic activity and high corrosion resistance in theelectrolysis of aqueous solutions, three of the present inventorsprepared amor-phous alloys and supersaturated solid solution alloyscontaining small amounts of platinum group metals, and treated them foractivation by utilizing the property that an amorphous alloy orsupersaturated solid solution alloy is very highly homogeneous, and theyfound highly active and highly corrosion resistant electrode materialsto be used for eiectrolyzing aqueous solutions. For these materials, anapplication for patent registration of oxygen generating electrodematerials for the electrolysis of aqueous solutions was filed underJapanese Patent Application No. 85-123111, and applications for patentregistration of chlorine generating electrode materials for theelectrolysis of sodium chloride aqueous solution were filed underJapanese Patent Application Nos. 85-169764, 85-169765, 85-169766 and85-169767. Furthermore, three of the present inventors studied for thepurpose of obtaining electrode materials effective for electrolyticoxidation of methanol by utilizing the properties of amorphous alloys,and found that an amorphous (Ni and/or Co) - valve metal (Ti, Zr, Nband/or Ta) alloys containing a small amount of Pt, if necessary furthercontaining other platinum group elements than Pt, and Ti, Si, Ge, Pband/or Bi for assisting the action of Pt are highly active metalelectrodes for the electrolytic oxidation of methanol if the amorphousalloy is immersed in hydrofluoric acid. An application for patentregistration of the materials was filed under Japanese PatentApplication No. 86-154570.

Based on the results of these studies, the present inventors studied onthe catalysts which can convert exhaust gases generated by combustion offossil fuels to harmless gases, even at temperatures close to roomtemperature, and found that high performance catalysts can be obtainedfrom similar amorphous alloys. Thus, the present invention has beenachieved.

The present invention comprises claims 1 to 3, and provides catalystshigh in activity and easy to produce and recover, obtained byhydrofluoric acid treatment of amorphous alloys consisting of Co or Niand valve metals, and further containing small amounts of platinum groupelements acting catalytically for conversion of exhaust gases toharmless gases.

Table 1 shows the component elements and contents stated in claims 1 to3.

                  TABLE 1                                                         ______________________________________                                        Compositions of alloys of the present invention (at %)                                                  Ru, Rh, Pd                                          Claim Ta, Nb (*1)                                                                             Ti, Zr (*2)                                                                             Pt, Ir (*3)                                                                           Ni, Co (*4)                                 ______________________________________                                        1     20-70               0.5-20  Balance                                     2               20-80     0.5-20  Balance                                                                       (but 10 or more)                            3     At most 70                                                                              20-80 (*5)                                                                              0.5-20                                                                                (but 10 or more)                            ______________________________________                                         (*1). At least one element selected from Ta and Nb                            (*2). At least one element selected form Ti and Zr                            (*3). at least one element selected from Ru, Rh, Pd, Ir and Pt                (*4). The balance being substantially at least one element selected from      Ni and Co.                                                                    (*5). The sum of at least one element selectef from Ti and Zr and at most     70 at % of at least one element selected from Ta and Nb                  

To obtain a catalyst with selectively high catalytic activity forspecific chemical reactions and easy to produce and reproduce, it ismore convenient to use an alloy containing required amounts of effectiveelements than platinum group elements carrying alumina, titania orsilica. However, a crystalline metal prepared by any ordinary method tocontain large amounts of various alloying elements forms a multiphasestructure with heterogeneous chemical properties, and hardly showspecific properties of solid solution. In addition, because ofbrittleness, it is difficult to obtain a material with a large specificsurface area required for a catalyst.

On the contrary, an amorphous alloy of the present invention obtained byrapid quenching of a molten alloy composed as described above consistsof a simple phase solid solution and has excellent mechanical propertiessuch as high strength and toughness, since the solid phase is inprinciple, formed by rapid quenching without permitting the localizationof component elements. If it is immersed in hydrofluoric acid foractivation treatment, the elements not so effective for catalyticactivity such as valve metals, nickel and cobalt are dissolved intohydrofluoric acid, with a consequent accumulation of the platinum groupelements with high catalytic activity in the surface. In this case,since the amor-phous alloy is a homogeneous solid solution, the platinumgroup elements, which are homogeneously distributed in the alloy, act ascathodes on which hydrogen is evolved violently in hydrofluoric acid.Hydrogen evolution assures the dissolution of the elements not soeffective for catalytic activity. Therefore, in an amorphous alloycontaining homogeneously dissolved platinum group elements, thedissolution of the elements not so effective for catalytic activityoccurs homogeneously and quickly, and thus, a highly active catalyst,which has a large surface area and contains concentrated platinum groupelements effective for catalytic activity, can be quickly produced.

Consequently, a catalyst, which can act even at relatively lowtemperatures close to room temperature for conversion of exhaust gasesto harmless gases, can be realized by an amorphous alloy catalyst of thepresent invention obtained after activation by immersion of the alloycomposed as above in hydrofluoric acid.

The reasons for limiting the respective ingredients in the presentinvention are stated below.

Ni and Co are basic elements of the alloys of the present invention andform an amorphous structure in the coexistence with one or more of Ti,Zr, Nb and Ta. In the first version of the present invention in which Niand/or Co and Ta and/or Nb coexist, an amorphous structure can be easilyobtained if the amount of Ta and/or Nb is 20 to 70 at %. In the secondversion of the present invention in which 10 at % or more of Ni and/orCo and Ti and/or Zr coexist, an amorphous structure can be easilyobtained if the amount of Ti and/or Zr is 20 to 80 at %. Furthermore, inthe third version of the present invention in which Ti and/or Zr and Nband/or Ta exist together with Ni and/or Co, an amorphous structure canbe easily obtained if the total amount of Nb and/or Ta and Ti and/or Zris 20 to 80 at % with the amount of Nb and/or Ta kept at 70 at % orless.

Platinum group elements, Ru, Rh, Pd, Ir and Pt give catalytic activity,but if they are contained too much, the catalyst becomes expensive, anddissolution of unrequired elements by hydrofluoric acid treatmentbecomes difficult. The excessive addition of platinum group elements,therefore, makes it difficult to increase the surface area and toconcentrate platinum group elements on the surface by hydrofluoric acidtreatment. Therefore, in the present invention, the amount of platinumgroup elements must be kept in a range from 0.5 to 20 at %, preferably 1to 10 at %.

For preparating an amorphous alloy of the present invention, the alreadywidely used method of forming an amorphous alloy by rapidly quenching ofa liquid alloy is used.

An apparatus for preparing the amorphous alloys of the present inventionis shown in FIG. 1 as an example. The space surrounded. by the dottedline is evacuated and filled with an inactive gas. In FIG. 1, symbol 2denotes a quartz tube with a vertical nozzle 3 at the bottom end. Froman inlet 1 at the top of the quartz tube 2, a raw material 4 and aninactive gas for preventing the oxidation of the raw material can be fedin. To heat the raw material, a heating furnace 5 is installed aroundthe quartz tube 2. A roll 7 rotated at a high speed is placed verticallybelow the nozzle 3, and is rotated by a motor 6. For preparing of anamorphous alloy, the raw material 4 with a predetermined compositon isput into the quartz tube 2, and the apparatus is evacuated to about 10⁻³Torr and is subsequently filled with an inactive gas. The raw material 4is heated and molten by the heating furnace 5, and ejected onto thecircumferential surface of the roll 7 rotated at a high speed of 1000 to10000 rpm by the motor 6. By this method, for example, an amorphousalloy of the present invention can be obtained as a long thin sheet of0.1 mm in thickness, 10 mm in width and several meters in length.

EXAMPLE 1

Raw alloys were prepared by mixing elements to achieve compositions asshown in Table 2 and melting by an argon arc melting furnace. Thesealloys were remolten in argon atmosphere and tepidly solidified usingthe single roller quenching method shown in FIG. 1, to obtain thinamorphous alloy sheets of 0.01 to 0.05 mm in thickness, 1 to 3 mm inwidth and 3 to 20 mm in length. The formation of amorphous structureswas confirmed by X ray diffraction. These alloy samples were immersed inis 46.5% hydrofluoric acid for 300 to 900 seconds, to obtain highlyactive metallic catalysts. A quartz tube of 8 mm in inner diameter wasfilled with 0.5 g of any of the catalysts obtained like this to a lengthof 5 cm, and installed as a reactor tube in an electric furnace.Nitrogen gas containing 243 ppm, by mole ratio, each of NO and CO wasfed into the reactor tube at a flow velocity of 100 mm/min, and theamounts of CO and NO at the outlet of the reactor tube were measured bya gas chromatograph and NO meter respectively. The results obtained arealso shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Conversion of gas containing NO and CO                                                            Temperature at which                                                          100% NO and CO were                                       Alloy               converted into N.sub.2 and CO.sub.2                       (at %)              (°C.)                                              ______________________________________                                        Ni--30Ta--2RH               165                                               Ni--30Ta--2Pt               175                                               Ni--30Ta--2Ir               190                                               Ni--30Ta--2Pd               240                                               Ni--30Ta--2Ru               250                                               Ni--30Ta--3Rh               190                                               Ni--40Nb--2Rh               170                                               Ni--40Nb--2Ru               250                                               Ni--40Ta--30Nb--10Rh        200                                               Ni--10Ta--10Nb--0.5Rh       230                                               Ni--20Ta--20Nb--10Rh--10Ru  270                                               Ni--20Co--30Ta--10Nb--2Rh   170                                               Co--40Nb--3Rh               190                                               Ni--70Ti--0.5Ru             260                                               Ni--40Zr--0.25Rh--0.25Pt    210                                               Ni--20Zr--1Pt               175                                               Ni--40Ti--40Zr--3Ir         190                                               Co--20Zr--5Pd               250                                               Co--20Ti--20Pd              260                                               Ni--20Co--40Zr--1Pd--1Rh--1Ru--0.5Pt                                                                      180                                               Ni--70Ta--10Ti--1Pd         250                                               Ni--10Ta--30Nb--20Zr--1Pd   260                                               Ni--10Ta--10Nb--20Ti--20Zr--3Ru                                                                           270                                               Co--30Nb--10Zr--3Ir         200                                               Ni--30Co--10Ta--10Nb--10Ti--10Zr--0.25Ir--0.25Rh                                                          190                                               ______________________________________                                    

EXAMPLE 2

Raw alloys were prepared by mixing raw metals to achieve compositions asshown in Table 3 and melting by an argon arc melting furnace. Thesealloys were remolten in argon atmosphere and rapidly solidified usingthe single roller quenching method shown in FIG. 1, to obtain thinamorphous alloy sheets of 0.01 to 0.05 mm in thickness, 1 to 3 mm inwidth and 3 to 20 mm in length. The formation of amorphous structureswas confirmed by X ray diffraction. These alloy samples were immersed in46.5% hydrofluoric acid for 300 to 900 seconds, to obtain highly activemetallic catalysts. A quartz tube of 8 mm in inner diameter was filledwith 0.5 g of any of the catalysts obtained like this to a length of 5cm, and installed as a reactor tube in an electric furnace. Nitrogen gascontaining 0.505 mole % of CO and 0.489 mole % of O₂ was fed into thereactor tube at a flow velocity of 33 mm/min, and the amounts of CO andO₂ at the outlet of the reactor tube were measured by a gaschromatograph. The results obtaiend are also shown in Table 3. Thesealloy catalysts could be very easily taken out of the reactor tubes, andcould be regenerated without any trouble after they had beencontaminated on the surface by actual reactions.

                  TABLE 3                                                         ______________________________________                                        Conversion of gas containing CO                                                                    Temperature at which                                                          100% CO was converted                                    Alloy                into CO.sub.2                                            (at %)               (°C.)                                             ______________________________________                                        Ni--30Ta--1Rh                205                                              Ni--30Ta--2Ru                190                                              Ni--30Ta--3Ru                180                                              Ni--30Ta--2Rh                140                                              Ni--30Ta--3Pd                110                                              Ni--30Ta--1Ir                110                                              Ni--30Ta--2Ir                140                                              Ni--30Ta--3Ir                130                                              Ni--30Ta--2Pt                105                                              Ni--40Ta--30Nb--10Rh         170                                              Ni--10Ta--10Nb--0.5Rh        160                                              Ni--20Ta--20Nb--10Rh--10Ru   200                                              Ni--20Co--30Ta--10Nb--2Rh    160                                              Co--40Nb--3Rh                160                                              Ni--70Ti--0.5Ru              180                                              Ni--40Zr--0.25Rh--0.25Pt     140                                              Ni--20Zr--1Pt                110                                              Ni--40Ti--40Zr--3Ir          110                                              Co--20Zr--5Pd                120                                              Co--20Ti--20Pd               120                                              Ni--20Co--40Zr--1Pd--1Rh--1Ru--0.5Pt                                                                       130                                              Ni--70Ta--10Ti--1Pd          120                                              Ni--10Ta--30Nb--20Zr--1Pd    130                                              Ni--10Ta--10Nb--20Ti--20Zr--3Ru                                                                            200                                              Co--30Nb--10Zr--3Ir          130                                              Ni--30Co--10Ta--10Nb--10Ti--10Zr--0.25 Ir--0.25 Rh                                                         110                                              ______________________________________                                    

As can be seen from the above, the amorphous alloy catalysts of thepresent invention for conversion of exhuast gases to harmless gasesallow exhaust gases to be converted at very low temperatures, and havevery high activity.

As described above, even though the amorphous alloys of the presentinvention are very low in the content of expensive platinum groupelements, the highly active catalysts prepared by treating them byhydrofluoric acid exhibit very high catalytic activity, and thus havehigh catalytic activity to be able to convert exhaust gases to harmlessgases at low temperatures.

Furthermore, the highly active catalysts of the present invention areprepared by hydrofluoric acid immersion of an amorphous alloy ribbonformed by any method rapid quenching from the liquid state such assingle roller quenching method generally used for preparation ofamorphous alloys. Therefore, no special apparatus is required.

Thus, the preparation of highly active catalysts of the presentinvention does not require any specially complicated or expensiveoperation, and the highly active catalysts thus prepared of the presentinvention have excellent catalytic activity and can be easilyregenerated, being practically excellent.

We claim:
 1. Catalysts for conversion of exhuast gases to harmlessgases, comprising an amorphous alloy consisting of 20-80 at % of atleast one element selected from Ti and Zr, 0.5-20 at % of at least oneelement selected from Ru, Pd, Rh, Pt and Ir, with the balance beingsubstantially 10 at % of at least one element selected from Ni and Co,having been subjected to activation treatment by immersion inhydrofluoric acid.